Shape tuna mixture into 6 (3/4-inch-thick) patties. Cover and refrigerate for 30 minutes, or until chilled and firm. Coat grill pan with spray. Grill 2–3 minutes or until bottom is well browned. Turn patties; cook 3–4 minutes more or until browned and 145°F.

Spread mayonnaise evenly over both cut sides of each bun; add tuna patty. Top with red cabbage slaw and bun tops. Serve immediately with remaining slaw.